You're watching a video or in the middle of a game and a notification slides in over the top, blocking what you're doing. Those are heads-up (or "peeking") notifications. You can kill the interruptions without muting the app completely — so you still get the alert, just quietly in the shade.

What these pop-ups are

Heads-up notifications are the floating banners Android shows for high-priority alerts (calls, alarms, sometimes chat pings). They're useful in theory but noisy in practice when every app claims "urgent". The fix is to lower an app's priority so it stops peeking.

Stop one app's pop-ups

Turned off pop-ups but still miss the important stuff? Dingpin pins key notifications to the top of your shade so they're always visible the moment you look — no pop-up needed.
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Silence all pop-ups

Why some still pop